Category: physics

The Ultraviolet Catastrophe

Post author By Ngtriplett
Post date June 4, 2020
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So what comes to mind when you hear the term catastrophe? It certainly sounds ominous and dreadful. According to merriam-webster.com a catastrophe is: a momentous tragic event ranging from extreme misfortune to utter overthrow or ruin and/or utter failure. It is the “utter failure” description that more accurately describes this particular catastrophe. So just what is the ultraviolet catastrophe and what was its significance to the birth of quantum mechanics? Keep reading to find out.

As the 19th century drew to a close, there was among the scientific community, an belief that most of the physical phenomena could be described and explained through classical physics. Nature, however, had other ideas. A series of discoveries would usher in a new branch of physics that classical or Newtonian physics was not able to describe. The photoelectric effect was one of these discoveries. This phenomena was accurately described by Albert Einstein which demonstrated that light travels as a wave and interacts with matter as particles. This is often described as the dual nature of light. The double slit experiment showed that particles, such as electrons, could produce interference patterns by interfering with themselves as they traveled through a narrow set of slits. This remarkable experiment showed that the behavior of individual particles could travel as particles and still demonstrate wave like behavior. Oddly enough, when scientists attempted to determine which slit the particle traveled through the interference pattern collapsed. The result of this collapse is what would be expected from a series of single particles being fired though one slit or another. Both of these topics have been discussed in earlier blog posts on my site. Feel free to go read those posts if you haven’t already.

So both of the above experiments helped describe discrete amounts of fundamental units. These units were considered to be quantized that is containing the smallest whole number amount of a specific material. For example, a quanta of light is a photon. So let’s see how the ultraviolet catastrophe relates to all this.

Blackbody and Blackbody Radiation

It will be helpful to be familiar with some of the vocabulary associated with this phenomenon. A blackbody is an idealized or theoretical object that absorbs all electromagnetic radiation that falls on it. A blackbody, according to Kirchoff, is a body that is able to “…completely absorb all incident rays, and neither reflects nor transmit any.” A blackbody emits blackbody radiation which is also known as thermal radiation. Many objects such as people, heating elements, flames, and the sun approximate blackbodies.

So you may be wondering how would a person experimentally measure blackbody radiation? You could set up a hollow container with a small hole in it. This cavity allows incident radiation to get in while the design makes it unlikely that much radiation will escape.

A blackbody cavity courtesy of https://phys.libretexts.org/

A small amount of emitted radiation will pass back out through the cavity opening where it can be measured. Thermal equilibrium occurs when two objects are in direct contact or close contact with each other but no net energy is transferred between them. They may gain energy from one another but no net energy is transferred. The amount of emitted radiation is small enough that it will not disturb the thermal equilibrium inside the cavity. As it turns out, the radiation a blackbody emits depends upon the temperature of the body.

Catastrophe

In science a theory and the mathematical representation of the theory must match the experimental results in order for the theory to be valid. Using classical physics, Lord Rayleigh and Sir James Jeans, working independently developed a law to predict the amount of radiation intensity emitted at a given wavelength. They were trying to come up with a rule that would allow them to accurately predict the color a blackbody would radiate based on temperature. The Rayleigh-Jeans Law predicted that the intensity of radiation was proportional to the wavelength so as the intensity increased the wavelength should decrease.

Courtesy of researchgate.net

In the graph above you can see that as the temperature in Kelvin increases the peak of the curve is shifted to the left. At 5000 K the peak of the curve is in the red portion of the spectrum and at 6000 K the peak is in the yellow portion of the spectrum. The catastrophe occurred in the ultraviolet portion of the spectrum and it stated that at low wavelengths the intensity should become infinite. This obviously was a violation of the law of conservation and because this occurred in the ultraviolet portion of the spectrum it was named the ultraviolet catastrophe.

Courtesy of Professor Dave Explains You Tube channel

The graph above shows the experimental results versus the classical theory predictions regarding the intensity of radiation versus wavelength. You can see that at higher wavelengths the classical theory closely matches experimental results but at lower wavelengths there is no agreement.

A Quantum Way Out of Catastrophe

Max Planck solved the ultraviolet catastrophe in 1900. Planck developed a model in which the electromagnetic radiation in the cavity is absorbed by simple harmonic oscillators or resonators. These oscillators were merely a description and he didn’t argue that they actually exist in the walls of the cavity. Planck determined that the energy absorbed by these resonators needed to be in discrete packets or quantized. These discrete packets of energy became known as quanta of energy. This idea of quanta of energy was the birth of quantum theory. He developed a formula to relate the energy absorbed to the frequency of oscillation: E = hv where E is the energy absorbed, v is the frequency of oscillation and h is Planck’s constant 6.626 x 10^-34 J s

Max Planck derived his blackbody radiation law working backward from Wein’s distribution law. Planck realized that Wein’s distribution did not agree with experimental data at long wavelengths, that is wavelengths in the infared portion of the spectrum. The results from Planck’s blackbody radiation law are in agreement with experimental data.

The above equation was derived from Wein’s distribution law. Planck’s equation differs in that it allows the units of measure to be derived entirley based on the universal physical constants or the four fundamental constants of nature, Planck’s constant (h), Boltzman’s constant (kB), speed of light (C), and the universal gravitational constant (G).

Planck’s theoretical result (continuous curve) and the experimental blackbody radiation curve (dots). Courtesy of https://phys.libretexts.org/

Max Planck was able to work through the ultraviolet catastrophe and develop an theory that matched the experimental results. His realization that the radiation in the cavity must be absorbed in discrete quantities ushered in a brand new branch of physics called quantum mechanics. Here is a short video describing Planck’s discovery of light being quantized and the birth of quantum mechanis: https://youtu.be/i1TVZIBj7UA Ironically, many physics students probably refer to this as the quantum catastrophe!

Tags blackbody, blackbody radiation, Max Planck, quantum mechanics, Rayleigh-Jeans Law, ultraviolet catastrophe

astronomy physics solar system

Introducing Comet SWAN

On March 25th amateur astronomer Michael Mattiazzo discovered Comet SWANN (C/2020 F8) using the Solar and Heliospheric Observatory’s SWAN (Solar Wind Anisotropies) camera. (skyandtelescope.org) Viewers in the Northern Hemisphere should be able to view the Comet SWAN beginning in Mid May. The discovery of this previously unknown comet got me thinking a bit about these celestial objects. This comet has only been recently discovered so we don’t yet know much about it. I wanted to give a brief description of comets and talk a little about what we do know about these fascinating objects.

According to solarsystem.nasa.gov “Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun.” This certainly doesn’t seem like much to get excited about. Let’s look a little closer at these “cosmic snowballs”. Think back to your days in middle school and high school science class when you learned about atomic structure. One of the things that stood out to me is how similar each atom was varying only by number of electrons and/or neutrons. It was easy to draw an atom of any element using the Bohr model because they all had the same basic structure, a positively charged nucleus containing some protons and neutrons, and electrons with their negative charges orbiting the positively charged nucleus. It turns out that active comets which are close to the sun have all have similar structure as well, meaning they all have the same component parts but may vary from comet to comet in size, composition, and place of origin.

The Structure of Active Comets

The structure of active comets all contain several distinctive features. The nucleus of a comet is the portion which contains mostly ice and gas with trace amounts of dust and other particles and is generally 1 to 10 km in size. It is possible, however for a comet to have a nucleus of up to 100 km. The nucleus of an active comet is mostly in a solid state and fairly stable. The coma is a cloud of water, carbon dioxide and various other gases which are converted directly from solid to gas as they are heated as the comet approaches the sun. This part of the comet may be over a thousand times larger than the nucleus. The nucleus along with the coma form the head of the comet. The hydrogen cloud or envelope is yet another feature of comets, this is an extremely large cloud of hydrogen which envelopes the comet. This cloud may be in the vicinity of millions of kilometers in diameter. Active comets that are nearing the sun have two distinct tails. The dust tail, which is the most obvious and visible feature to the unaided eye, is composed of mainly dust and other gases and may be upwards of 10 million kilometers long. Interestingly, the dust tail always points away from the sun. The dust tail reflects light from the sun making it the most visible portion of the comet. The second tail, the ion tail is composed of plasma and other particles related to the comets interaction with the solar winds associated with the sun. This tail may be much longer than the dust tail reaching lengths of several hundred kilometers.

28+ [ Comet Diagram ] | schematic diagram of a typical comet the ... — Courtesy of http://0osr12g1.adtddns.asia/comet-diagram.html

The Path of a Comet around the Sun

Comets, like planets orbit around the sun. The orbit of a comet follows a much more elliptical path than do planets. The length of time it takes an object to complete one orbit is called a period. The period of the Earth’s rotation around the sun is 365.25 days and the period about its axis is 23 hours and 56 minutes. The period of the moon is 27 days which is the amount of time it takes to orbit the Earth. Comets in general have much longer periods. Any comet with a period of less than 200 years is referred to as short period comets. These comets originate from the Kuiper Belt. In case you were wondering, the Kuiper Belt is “a donut-shaped region of icy bodies beyond the orbit of Neptune. (solarsystem.nasa.gov) If the period of a comet is greater than 200 years it is called a long period comet and originates from the Oort cloud which is “is believed to be a giant spherical shell surrounding the rest of the solar system and is the most distant region of our solar system. Even the nearest objects in the Oort Cloud are thought to be many times farther from the Sun than the outer reaches of the Kuiper Belt.” (solarsystem.nasa.gov) Halley’s comet is perhaps one of the most famous comets of all time and has a period of 76 years making it a short period comet. The Hale Bop comet is a long period comet with a period of over 2500 years.

An article featured on Space.com, author Joe Rao wrote about the period of comet SWAN where he stated “for fun, I fed its orbital elements, which includes the eccentricity of its path around the sun, into an orbital simulator. My simulation suggests Comet SWAN is traveling around the sun in a period of about 25 million years. (space.com) The discovery of this comet is so new that the actual period of orbit remains to be seen but this certainly would be a fascinating development. We would be the very lucky few of humankind to be able to view this very long period comet.

I've heard that the Oort Cloud contains trillions of icy bodies ... — Courtesy of astronomymagahttps://astronomy.com/magazine/ask-astro/2018/04/the-oort-clouds-icy-bodieszine.com

How to View Comet SWAN

The best time to view SWAN will be in the evening hours or just before dawn beginning on May 23rd until June 10th according to skyandtelescope.org. The comet currently is visible using binoculars as well as the unaided eye to those in the Southern Hemisphere. Curious stargazers wishing to view the comet with the unaided eye in the Northern Hemisphere shouldn’t have to wait much longer as it will be visible to you by late May. The comet will reach perihelion, which is its closest distance to the sun around May 27th. At perihelion the SWAN comet will be roughly 64 330 000 km from the sun and the closest it will be to Earth will be on May 12-13th when it will be a mere 83 330 000 km from Earth.

Stay tuned to your favorite astronomy magazine or website to learn more about this newly discovered comet. Michael Mattiazzo, the amateur who discovered this comet has discovered a total of 8 comets dating back to 2004. Keep your feet on the ground, eyes open and head to the heavens and maybe you will discover the next comet or other astronomical object.

Tags Comet SWANN, comets, Hale Bopp comet

physics

The Photoelectric Effect

A Brief History of the Photoelectric Effect

In 1887 scientist Heinrich Hertz discovered the photoelectric effect while experimenting with a device called the spark gap generator which is a precursor to the radio. What Hertz found while using this device was “…sparks generated between two small metal spheres in a transmitter induce sparks that jump between between two different metal spheres in a receiver.” (https://physics.info/photoelectric/) The sparks that were jumping across the gap were, in fact electrons, which were receiving energy from ultraviolet light. In other words “when ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place.” (https://www.britannica.com/science/photoelectric-effect) This was an important finding because up until this point people where unaware of the relationship between light and electricity.

JJ Thompson, who many remember for his “plum pudding” model of the atom, discovered that the particles that were freed in the photoelectric effect were the same particles observed in the cathode rays he had been working with. His research using the cathode ray tube led to the discovery of corpuscles which we now know as electrons.

A depiction of a cathode ray tube. Courtesy of study.com

In 1902 Philipp Lenard made a shocking discovery. He found that as the frequency of light increased so to did the energy of the electrons. The expected result was that as the intensity or brightness of light increased the energy of the electron would increase. The experimental observation did not match the accepted theory of the time. So what does all this mean and who could make sense of it all? Before we get to that let’s discuss what specifically the photoelectric effect is.

What is the Photoelectric Effect?

So we know a little about the history of the photoelectric effect and we know that it has something to do with electrons and the frequency of light. So what is it exactly? Great question, glad you asked. The photoelectric effect is a phenomenon which occurs when a light of a high enough frequency is shown onto a photo-sensitive metal resulting in the ejection of electrons from that metal. If the threshold frequency is not high enough then no electrons will be ejected. The threshold frequency varies for different metals and is the minimum frequency required for electrons to be ejected.

Increasing the intensity of the light will result in more electrons being ejected provide the frequency is at or above the threshold frequency. A high intensity light at a frequency below the threshold frequency will not result in the ejection of electrons, in other words the photoelectric effect will not be observed. Increasing the frequency of light results in an increase in kinetic energy of the photons. The intensity of light had no impact on the kinetic energy of the photons only on the number of photons being ejected.

Einstein and the Photoelectric Effect

Einstein is of course known for his theory of general and special relativity, his work with quantum mechanics, and his famous equation E=mc^2. What you might not realize is that Albert Einstein was awarded the 1921 Nobel Prize in physics for “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.” (https://www.nobelprize.org/prizes/physics/1921/summary/)

As you now know, Einstein was not the first to observe the photoelectric effect, so why is he so often credited and associated with it rather than Hertz, Lenard, or Thompson? He was the first to accurately describe how it occurs and to make the ground-breaking discovery relating waves and particles.

Einstein realized two facts regarding the photoelectric effect: 1) light is made of particles called photons and 2) the metal can only absorb the entire photon and not any other portion of it, think of it as an all or nothing proposition. Some of the energy of the photon that is absorbed is used to free the electron and the rest is converted to kinetic energy of the photon. The energy required to free the electron is called the work function. The strict definition of the work function is “energy (or work) required to withdraw an electron completely from a metal surface.” (https://school.eb.com/levels/high/article/electronic-work-function/32337)

What does all this mean? The results that Einstein observed were not in agreement with the accepted theory of the time. A new model of light was needed to match observation. Einstein had the insight to recognize that sometimes light acted as a wave and sometimes it acted as a particle. This was a stunning revelation that shocked the scientific community. Sir Isaac Newton thought light must be made of particles in order for it to experience reflection and refraction while Robert Hooke had argued that light had wave like behavior. Finally Einstein came along and settled the argument: light acts as both a wave and a particle.

Today we refer to this as the wave-particle duality or the dual nature of light. In general light travels as a wave and interacts with matter as particles called photons. Light is quantized meaning it is packaged in discrete units or particles which are called photons. In 1900 Max Planck, the father of quantum mechanics derived the equation for the energy of electromagnetic radiation, including light. Here is a short video describing Einstein and his contributions to the photoelectric effect: https://youtu.be/0b0axfyJ4oo

If we use the above equation and compare it to the known work function of a specific metal we can determine if the photoelectric effect will occur. The energy of the photon must be greater than the work function in order for the photoelectric effect to be observed. Interestingly enough, the electrons that were ejected from the metal end up falling back into the metal almost immediately.

What Did We Learn from the Photoelectric Effect?

We now know some of the history of the photoelectric effect and what the photoelectric effect is all about. Let’s take a moment and summarize what we learned from this scientifically significant discovery. Most importantly we leaned that classical physics can not accurately predict what happens at the atomic level. Classical physics predicted that increasing the intensity of light should increase the energy of the photons. What actually happened was that increasing the frequency of light resulted in an increase in the energy of a photon while increasing the intensity of light only resulted in an increases in the number of photons being ejected. Einstein was able to determine the energy of a photon by the equation E=hv and that the energy of the photon must be greater than the work function in order to be ejected from the metal. The photoelectric effected demonstrated the dual nature of light, that is it has both wave and particle behavior, in general light travels as a wave and interacts with matter as particles.

Tags Albert Einstein, cathode ray tube, electron, Heinrich Hertz Philipp Lenard, JJ Thompson, Max Planck, photoelectric effect, quantum mechanics, Robert Hooke, threshold frequency

astronomy cepheid variable physics solar system supernova

How Do We Measure Distances in Space: Cosmic Distance Ladder Part 2

Post author By Ngtriplett
Post date April 10, 2020
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Last week we discussed several rungs on the cosmic distance ladder including stellar parallax, spectroscopic parallax, and main sequence fitting of stars. We discussed several astronomical terms associated with the cosmic distance such as astronomical unit, light year, and parsec. That discussion got us about halfway up the cosmic distance ladder. Lets ascend the rest of the way.

Variable stars

Have you ever wondered if the brightness of stars ever change periodically? There is a class of stars called Cepheid Variable stars that do just that. According to the NASA website starchild.gsfc.nasa.gov/docs/StarChild/questions/cepheids.html, Cepheid Variables “are stars which brighten and dim periodically. In 1912 a Swiss scientists made a remarkable discovery regarding 25 Cepheid Variables located in the Magellanic cloud. Henrietta Swan Leavitt noticed that these stars seemed to brighten and dim at regular predictable intervals called periods. She postulated that the brighter the Cepheid, the longer the period. As it turns out once you know the period of one of these variables the brightness can be inferred.

Cepheid Variables fall into two general classes: the first are those of a period of pulsation from about 1.5 days to 50 days. These population 1 stars are often found in the spiral arm of galaxies. The second class of Cepheid Variables are Population 2 and are “much older stars less luminous, and less massive than their Population I counterparts.” These are stars “…with periods greater than about 10 days and BL Herculis stars with periods of a few days.” https://www.britannica.com/science/Cepheid-variable

So how do these stars which brighten and dim in predictable patterns help us determine the distance to nearby galaxies? The apparent magnitude of these variables can be plotted at different times to develop a light curve. A light curve is a relationship between brightness and time. Using the information from the light curve and data collected using sensitive photometric equipment, the apparent magnitudes and period of the star can be determined. These values can be plotted on a “period-luminosity” graph to determine its absolute magnitude. Once this value has been obtained you use the distance modulus formula to determine the distance to the Cepheid Variable.

Standard Candles

Cepheid Variables and other astronomical objects, such as supernova, which have a known luminosity across the entire class of objects can be used to determine the distance of nearby galaxies as well as the expansion of the universe. These objects are collectively referred to as standard candles. According to http://planetfacts.org/standard-candle/ “A standard candle is a class of astronomical objects that belong to the same class and have a standard luminosity or brightness. You can actually determine an object’s distance from the earth using standard candles.” This method of distance measurement works like this according to planetfacts.org “…. a technical process which involves comparing the object’s brightness against a known or measured brightness from objects that belong to the same class. For example, you spot a certain object like a star or supernova, and determine that it is a standard candle; you can get its distance by measuring its brightness and comparing it to the known brightness of objects that are similar to it.” In our discussion regarding Cepheid Variables we learned about two different classes of these types of variables. If astronomers were trying to determine the distance of a far away galaxy, for example, and that galaxy had a Population 1 variable in it they could then compare its brightness to a Population 1 variable with a known distance to determine the distance of the galaxy.

The Tully-Fisher Relationship

We saw that by using variables and supernova as standard candles we could determine the distance astronomical objects. Is there a method to use the object which we want to study to determine how far away it is? Well, yes actually. The Tully-Fisher relation is just such a method. According to www.noao.edu/staff/shoko/tf.html the Tully Fisher relation is “a correlation for spiral galaxies between their luminosity and how fast they are rotating.” Scientists know that larger galaxies rotate with greater velocity then smaller galaxies.

Courtesy of spiff.rit.edu/classes/ladder/lectures/distant_gal/distant_gal.html#tf

“The key point of the Tully-Fisher relationship is that the speed of rotation of material in a spiral galaxy is related to the luminosity of that galaxy: high speeds occur in galaxies of high luminosity.” Again we see that by comparing the absolute magnitude against the apparent magnitude the distance of a given galaxy can be determined. The Tully-Fisher Relationship allows us to measure the distance of galaxies up to hundreds of megaparsecs away.

Hubble’s Law

We have now reached the top ring of the cosmic distance ladder. Hubble’s Law can be used to determine distances to the edge of our universe. “What is Hubble’ law” is a question I hope you are asking. Glad you asked, Hubble’s law, according to http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/hubble.html#c1“is a statement of a direct correlation between the distance to a galaxy and its recessional velocity as determined by the red shift. So what exactly is red shift?

You may have the heard the term “Doppler” when watching the weather on your local news. The term Doppler shift, with respect to light waves is an indication that the source emitting the waves is either moving toward or away from an observer. If the source is moving toward the observer the frequency of the waves increases as the waves bunch up toward each other. A result of this increased frequency is that the light is blue shifted, meaning the color is shifted toward the blue end of the spectrum. Bluish colors have higher frequencies than do reddish colors. If the source of light is moving away from an observer the light is shifted toward the red end of the spectrum as the frequencies are reduced. Here is a good explanation of Doppler shift both for sound and light waves: https://www.youtube.com/watch?v=h4OnBYrbCjY&feature=youtu.be

Fantastic, now we know about the Doppler shift but what does this have to do with Hubble’s law? In 1929 Edwin Hubble made a shocking discovery. He was able to determine that every other galaxy was flying away from us. Perhaps even more astonishing was the fact that the farther away a galaxy was the faster it was moving away from us. The law which bears his name states that the velocity (v) of a galaxy is equal to the product of the Hubble constant (H) * the distance to a distant galaxy. This relationship is generally used to find the distance of galaxies over a billion light years away.

The velocity of the galaxy is a measure of the recession rate of the galaxy, in other words how fast the galaxy is flying away from us. The Hubble constant has an estimated value of 70 kilometers per second per megaparsec. Not only can this equation be used to determine how far away and how fast galaxies are moving from us but this equation was used to prove that the universe was in fact expanding. So how was Edwin Hubble able to determine that these galaxies were moving away from us, well he was able to determine a red shift of each of the galaxies he observed. Hubble’s law has also contributed to many of astronomy’s deepest and most fascinating issues including providing the “first concrete support for Einstein’s theory of Relativity… It also helps validate theories of Dark matter and Dark energy. A recent discovery in 1998 revealed that the expansion of the universe is accelerating.” this according to planetfacts.org/hubbles-law/

Well I hope these two posts hope clear up how astronomers determine the distance of objects from our own solar system out to the edge of our universe. As technology continues to evolve and develop I would expect that new and improved ways of measuring cosmological distances will soon be devised. Maybe someday technological advances will allow us to go and explore the universe up close and in person.

Tags The Tully-Fisher Relationship

astronomy cepheid variable physics solar system

How Do We Measure Distances in Space: The Cosmic Distance Ladder Part 1

Post author By Ngtriplett
Post date March 28, 2020
1 Comment on How Do We Measure Distances in Space: The Cosmic Distance Ladder Part 1

Part 1

This will be a 2 part discussion about the Cosmic Distance Ladder. Part 1 will cover the Parallax method, Spectroscopic parallax, and Main sequence fitting as methods to determine the distance of stellar objects from Earth. I will begin by defining some astronomical units of measure and then discuss the techniques used to measure the distance of objects throughout our Solar System, the Milky Way galaxy, and the Universe.

Part 2 will focus on the use of variable stars, such as Cepheids and RR Lyraes, as well as nearby galaxies. We will also explore The Tully-Fisher Relationship, and Hubble’s Law to determine the distance of objects deep in our universe. I hope that after reading part 1 & 2 you will have a better understanding of how the distance of objects from our Solar System to the ends of the Universe are determined.

The moon is roughly 250,000 miles from Earth, the sun is 93 million miles from the Earth or 1 astronomical unit away (AU). The next nearest star to the Earth, Proxima Centauri is 4.3 light years (ly)away. The Milky Way Galaxy is 30,000 parsecs (pc) or 30 kiloparsecs (kpc) measured from one side to the other. What do some of these units of measure such as light year, astronomical unit, and parsec mean? How do we actually know how far these objects are from Earth or how far apart they are from each other?

Astronomical Units of Measure

1 AU of astronomical unit is equal to the average distance between the Earth and Sun which is 149,598,000 km (92,955,887 miles.) This unit is used to describe distances within our solar system. Neptune, for example, is 30.1 astronomical units from Earth. This equates to about 2,700,000,000 miles. It is much easier to to calculate using 30.1 au then it is to use 2.7 billion miles.

1 ly or light year is the distance light travels in a year. It may sound as if it is a measure of time but it is actually a measure of distance. 1 light year is equal to 5,878,630,000,000 miles. It is much easier to think and calculate using light years when dealing with the enormous distances associated with the universe.

The Hertzsprung-Russell diagram or HR diagram is “… a graph that is used for making stellar calculations. It gives the relationship between the luminosity of stars against their type and temperature” according to https://universavvy.com/astrophysics-hertzsprung-russell-diagram-explanation. This diagram can be used to classify, trace the life cycle, as well as teach about the how stars work.

An HR diagram plotting luminosity v temperature. Courtesy of mtholyoke.edu

Stellar Parallax and the Parsec

In order for our discussion of parsec to make sense I am going to discuss the phenomena of parallax first and then tie it into our description of a parsec. On the cosmic distance ladder, parallax is the only direct method we have to measure distances to stars. To give you an easy and quick example of parallax hold one arm straight out in front of you and point your thumb to the sky. Now alternate opening and closing your right and then your left eye. The apparent movement of your thumb is an example of parallax.

Astronomers can use this idea to measure the distance of a star. In the image below you can see the position of the Earth six months apart. The angle of parallax can be measured by comparing how the star has shifted as compared to the stars behind it. You can see that a triangle has been formed between the star and the two positions of the Earth six months apart. By using some trigonometry you can determine the distance to the star. This method of measuring distance is limited because the parallax angle for more distant stars is to small for us to measure.

1 pc or parsec is equal to 3.26 ly or 206,280 AU. I know what you are thinking, Han Solo said about the Millennium Falcon “it’s the ship that made the Kessel run in less than 12 parsecs.” https://youtu.be/fjYuw6zWk_Y Maybe “in a galaxy far, far away” a parsec is a measure of time, but in our galaxy it is a measure of distance. The definition of 1 parsec is the distance to an object which has a Parallax angle of 1 arscencond. You can see the term parsec is a combination of parallax and arcsecond. To measure the distance, in parsecs, using parallax angle you use the equation d = 1 AU/p” where p is the parallax angle. Your answer will be the distance in parsecs. This method of calculating distance only works for stars that are within roughly 300 parsecs or 978 light years from Earth, beyond that distance the parallax angles are to small to measure.

Spectroscopic Parallax

According to https://www.atnf.csiro.au/outreach/education/senior/astrophysics/photometry_specparallax.html “The term spectroscopic parallax is a misnomer as it actually has nothing to do with parallax. It is, however, a way to find the distance to stars. Most stars are too far away to have their distance measured directly using trigonometric parallax but by utilising spectroscopy and photometry an approximate distance to them can be determined.”

So then what is spectroscopic parallax and how does it help us determine the distance of stars? Astronomers determine the spectral and luminosity class of a given star. Using photometric equipment the apparent magnitude and color index can be determined. The star is placed on to the HR diagram and its apparent magnitude is deduced. At this point an equation called the distance modulus equation is used to determine the distance in parsecs of the star. According to Australia National Telescope Facility “In practice this technique is not very precise in determining the distance to an individual star…..Nonetheless it is still an important methods for estimating distance to stars beyond direct trigonometric parallax measurement.”

Main Sequence Fitting

Main sequence fitting as a method of determining cosmic distance is done by comparing the relative distance of open clusters to the distance of the Hyades galaxy which has a known distance. An open cluster according to https://www.sciencedaily.com/terms/open_cluster.htm is “a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other.” The stars in these clusters are believed to have been formed at the same time and located the same distance away from Hyades. The distance of Hyades was found via direct measurement using stellar parallax.

Scientists will measure the spectra of the cluster to determine “spectral type, luminosity, and temperature and define a main sequence” according to Teach Astronomy https://www.youtube.com/watch?v=hq29cjeR2o4. Scientists compare the shift between the open cluster and the Hyades galaxy on the HR diagram and use the inverse square law to determine the relative distance between the open cluster and the Hyades galaxy. The inverse square law states that the the light will get dimmer by a factor of the square of the distance away from the source of light.

Next week we look at part 2 of the Cosmic Distance Ladder which should give you a fuller picture of how astronomers determine the distance of objects throughout our universe.

Tags cosmic distance ladder, cosmology, HR diagram, parallax

physics thermal physics thermodynamics

A Brief Conceptual Introduction to Entropy

Post author By Ngtriplett
Post date March 18, 2020
2 Comments on A Brief Conceptual Introduction to Entropy

A common depiction of entropy. Courtesy of chemistry.tutorvista.com

Disclaimer

I want to make it clear that this post is meant as a conceptual introduction to an often misunderstood topic. I do not go into the mathematics associated with entropy, nor do I discuss the contributions of perhaps the most crucial person in the history and development of entropy, Ludwig Boltzmann. This discussion is meant to introduce the reader to the basic terminology, including the laws of thermodynamics as well as the concept of entropy.

If I ask you the following question: “what is entropy,” how would you answer? Would you describe it as the tendency of a system to move towards disorder or chaos? This seems to be the oversimplified definition that many people have learned. I’d like to offer a different description of entropy, one that is a bit more in-depth and one that is more accurate. This idea of an increase in disorder is not altogether incorrect; it is somewhat incomplete. I hope to give you a conceptual picture of entropy while avoiding the cumbersome math involved.

Thermal Physics Vocabulary and The Laws of Thermodynamics

We need to discuss some topics of thermodynamics in order to more accurately understand entropy. I’d like to introduce some common vocabulary terms associated with thermal physics. These terms are presented in no particular order. Kinetic energy is the energy of a body in motion. The greater the speed of a particle or particles, the greater the kinetic energy. For our purposes, temperature can be described as the average kinetic energy of an object. Meaning the average kinetic energy of the particles at the microscopic level. As particles are heated up, they move faster and collide more often with each other than when the particles are at a lower temperature. Thermal equilibrium means that two objects that are in contact with one another have reached the same temperature. Thermal energy is simply heat. Absolute zero is a temperature so low that there is no heat is available to transfer to other systems. Sciencedaily.com describes absolute zero as “the point at which the fundamental particles of nature have minimal vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion.” Absolute zero is measured a 0 on the Kelvin scale, which corresponds o -273 on the Celsius scale and -460 on the Fahrenheit scale.

temperature scales — A comparison of the three temperatures scales. Courtesy of britannica.com

Let’s look at the terminology that is specific to the concept of entropy. A microstate, according to khanacademy.org, is “the arrangement of each molecule in the system at a single instant.” A macrostate is the observable configuration of the microstates. So what does that mean? Think of flipping 3 coins, labeled as A, B, &C. How can you end up with 2 heads and 1 tail? You can end up with an arrangement of heads for coin A, heads for coin B, and tails for coin C. You could end up with tails for coin A, heads for coin B, and heads for coin C, or you could end up with heads for coin A, heads for coin B, and tails for coin C. The exact configuration of which state (heads or tails) a specific coin lands on is the microstate. In the example I gave, you have a total of 3 microstates corresponding to a single macrostate. The macrostate is 2 heads and 1 tails. The microstate is the specific arrangement of the coins, as seen below.

In the above chart, you can see the specific 252 microstates that are associated with the microstate of a 10-coin system.

The Laws of Thermodynamics

The next topic worth discussing is the numbering system as well as the laws of thermodynamics. There are four laws, but they begin with the zeroth law and end with the third law. The zeroth law was actually included as a law much later than laws 1,2&3. “British physicist Ralph H. Fowler who first coined the term ‘zeroth law,’ based on a belief that it was more fundamental even than the other laws.” (https://www.thoughtco.com/laws-of-thermodynamics-p3-2699420) This zeroth law states that if two systems are in thermal equilibrium with a third, then they (the first two systems) are in equilibrium with each other. This is the basis of temperature and is actually how the old glass thermometers work. You place the thermometer under your tongue and wait for the thermometer, the patient, and the fluid in the thermometer to come to thermal equilibrium and take the temperature reading.

The first law of thermodynamics is a restatement of the law of conservation of energy. It states that energy in the universe can neither be created or destroyed; it can, however, be transformed from one form to another and be transferred to another object. An example of transformation from one form to another would be dropping a rock from a high cliff. Before the rock is dropped, it contains potential energy (think stored energy), and once it is dropped, the potential energy is transformed into kinetic energy (energy of motion) as the object falls to the ground. When the rock hits the ground, it transfers its energy to the ground.

According to https://www.grc.nasa.gov/WWW/k-12/airplane/thermo2.html, the second law of thermodynamics states “that if a physical process is irreversible, the combined entropy of the system and the environment must increase. The final entropy must be greater than the initial entropy for an irreversible process: Sf > Si (irreversible process)” This law tells us that the entropy of the universe is always increasing. There may be a local decrease in entropy in a system, but the entropy increase in the surroundings will always be greater than a local decrease.

The third law of thermodynamics states, according to http://physicsforidiots.com/physics/thermodynamics/, “As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.” This law is also used to illustrate that absolute zero is a temperature that is so low that it can never actually be reached.

So Now What?

So now we have these vocabulary terms and a set of laws, but how do they apply to entropy? First off let’s look at a definition of entropy, according to www.merriam-webster.com/dictionary/entropy entropy is “a measure of the unavailable energy in a closed thermodynamic system that is also usually considered to be a measure of the system’s disorder, that is a property of the system’s state, and that varies directly with any reversible change in heat in the system and inversely with the temperature of the system.” So is this the best working definition of entropy we can come up with? Short answer no.

Let’s take a look at what is really going on with thermodynamics and entropy and why the idea of “the tendency of a system to increase disorder” is not the best description of entropy. Think of a cup of black coffee which has liquid creamer poured into it.

It may appear that the creamer is in a highly disordered state, and as a result, you might predict that this cup of coffee would have greater entropy compared to a cup of coffee below.

The coffee cup in the first image does appear to be more disordered, but it is the coffee cup in the second image that has greater entropy. How can this be? If we change our working definition of entropy to be the tendency of a system toward the state with the maximum number of microstates, we can answer this question. In the first image of the coffee and creamer, as with a macrostate of 3 coins being heads, heads, heads, there are fewer ways in which the objects, molecules of creamer, or coins can arrange themselves for that specific configuration. If we look at the coffee with the creamer uniformly mixed with the coffee, we can see that there are more ways for the particles to reach uniform mixture because it doesn’t matter which particles of creamer mix with which particles of coffee in order to get the even mixture. Just as obtaining 2 heads and 1 tails is more likely than obtaining 3 heads because of the number of possible ways to obtain either outcome. There is only one way microstate that gives a macrostate of 3 heads, but there are 3 microstates that can deliver the macrostate of 2 heads and 1 tails.

Another example which illustrates that disorder does not necessarily equate to entropy is the universe. The entropy of the universe was very low after the big bang and had been increasing ever since. The universe moments after the big bang would have had little structure as compared to what we have now, galaxies, stars, planets, people. The kicker is that because of so many ordered and structured objects in the universe today, and there are many more microstates available to create all these objects.

The above are two examples why we need a better working definition for entropy than one that focuses on disorder. A more accurate or complete definition must include the idea of a system tending toward a state of the maximum amount of microstates for a given macrostate. There are many times when the old fashioned definition of chaos and disorder applies just fine, but it is not a completely accurate description.

Tags boltzmann, entropy, thermodynamics

astronomy Black holes physics

Black Holes

Most of us have some idea about black holes. How much you know about these objects likely varies on how interested you are in astronomy as well as your scientific background. Science fiction has long portrayed black holes as evil forces of nature bent on destroying mankind. Movies and books often portray these celestial bodies as monsters traveling across the universe eating up everything in its path. Based on these characterizations you might predict the end of our planet will most likely be due to an encounter with a rogue black hole. So before we start preparing for our demise via black holes lets all take a deep breath and discuss the facts about these things.

Escape Velocity

The first thing we should discuss is something that my students often recite whenever the topic of black holes is mentioned. They will automatically say “it is something so dense not even light can escape it” while this is correct what does it actually mean? In order for something, be it light or a rocket, to leave a planet or anything with a gravitational field the object must over come what is called escape velocity. So what does it actually mean? Escape velocity is the speed needed for an object to overcome the gravitational attraction of a planet, or other body. The gravitational attraction can be calculated by the following equation:

Image result for law of universal gravitation — Courtesy of sciencephoto.com

Where F is the gravitational attraction, G is the universal gravitational constant and is equal to 6.67 x 10^-11 m^3/kg * s^2, m1 and m2 represent the masses of the planet and the object being attracted to it and r^2 is the distance between the two objects. You can use this formula to calculate the gravitational attractive force between any two objects in the universe. In order to break free from the gravitational attraction an object must be traveling fast enough to overcome this force. Escape velocity can be calculated by the following formula:

Image result for escape velocity formula — Courtesy of getcalc.com

With this equation, which is derived from the kinetic energy formula and the Universal Law of Gravitation, you can calculate how fast something must travel to leave the planet. An object must be traveling at a rate of 11 km/s in order to leave the Earth. If, for example, one found them self on the surface of a neutron star the escape velocity would be 165,000 km/s.

So we have this equation why can’t we use it to calculate the escape velocity needed for light to leave a black hole? As it turns out this question was answered by Reverend Jon Mitchell in 1783. He was able to determine that, as a bizarre consequence of Newton’s law of motion, an object with the same density as the sun but 500 times its radius would have an escape velocity greater than the speed of light. So for any object, such as a black hole, that has an escape velocity faster than the speed of light it means nothing can escape that object. The more massive the object is and the smaller the object has been compressed the greater the escape velocity.

Features of Black Holes

Another feature of black holes is the event horizon. This is the boundary between the black hole itself and the space around it where objects may safely reside. Once an object has crossed over the event horizon the escape velocity exceeds the speed of light and therefore, no object can escape from it.

According to astronomy.swin.edu “The Schwarzschild radius is the radius of the event horizon surrounding a non-rotating black hole.” The equation for the Schwarzschild Radius is

Image result for schwarzschild radius — Courtesy of perthobservatory.com.au

Any object with a physical radius smaller than its Schwarzschild radius will become a black hole. This quantity was first derived by Karl Schwarzschild in 1916: The Schwarzschild Radius is the size an object must become in order for its escape velocity to exceed the speed of light. In order for the Earth to become a black hole it would need to be compressed down to the size of 0.9 cm or 1/3 of an inch and the sun would need to be compressed to down to 3 km or just under 2 miles. So as we will see later most stars in the sky do not become black holes at the end of their lives.

Image result for event horizon of a black hole — Courtesy of quora.com

There is a point inside the the black hole that general relativity tells us has infinite density and curvature this point is termed the singularity. According to Kip Thorne the singularity is “the point where all laws of physics break down”. Some scientists have suggested that an, as of yet, unknown combination of classical physics and quantum physics may be needed to more accurately describe the behavior and features inside black holes.

Often times black holes will be depicted with material swirling around it prior to being sucked into the black hole. This material is a combination of gas, dust, and other material making up what is called an accretion disk. Physicsoftheuniverse.com defines an accretion disk as “material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon…” We are able to see the accretion disk because the particles which make it up are spinning at a high rate of speed releasing heat, gamma rays and x-rays as the particles collide with each as a result of the black holes large gravitational force.

Image result for accretion disk of a black hole — Courtesy of svs.gsfc..nasa.gov This image depicts a black hole with an accretion disk spinning around it.

So How are Black Holes Formed?

Stellar black holes are formed when a star greater than 25 solar mass, that is 25 times more massive than the sun collapse in on themselves and goes supernova. When the internal pressure can no longer withstand the gravity, stars of this size collapse until all the matter is within the Schwarzschild radius. The resulting supernova ends up as a stellar black hole. These stellar black holes are roughly 3-100 solar masses or 3 to 100 times the mass of the sun.

A second way stellar black holes are formed is when neutron stars have exhausted their magnetic fields and devolve into small (2-5 solar mass black holes) black holes. These black holes are much smaller than those created by stars that are greater than 25 solar mass. As it turns out it is a relative rarity that stars become black holes upon their death. Most stars, including our own sun, are too small to wind up as stellar black holes.

Supermassive black holes are thought to be the at the center of galaxies including our very own Milky Way Galaxy. These black holes are 100,000 times the mass of the sun. Scientists believe that the enormous mass of these black holes is directly related to their location at the center of galaxies. These galaxies provide billions of stars and other material to feed the black hole so that it can grow quickly over time. Galaxies tend to collide with other galaxies which also allows for the supermassive black holes to grow even larger.

Image result for how are stellar black holes formed — Courtesy of steemit.com

Intermediate black holes which may range anywhere from 100 to 100,000 solar mass are created by the merger of other smaller black holes to create a single larger black hole or when a black hole “eats” large amounts of matter around it to increase its mass. Chain reactions caused by the collision of stars is another way that intermediate black holes are thought to be created.

Here is a video that combines many of the topics we’ve already discussed black hole features and how black holes are formed: https://www.youtube.com/watch?v=brmjWYQi2UM

How Do We Know Black Holes Exist?

Since we are unable to see a black hole, how do we know they exist? A fair and interesting question to be sure. Well, we can detect their effects on nearby objects such as gas and stars. Stars that orbit a black hole have a characteristic “wobble” when we observe them. The wobble is caused by a massive object exerting a gravitational pull on the object. The object must be very massive to exert a noticeable effect on its companion star. Scientist can estimate the mass of the black hole by the effect it has on its visible companion. We are also able to detect X-rays from the discs of gas falling into a black hole. This may happen when material from a companion star fall into a black hole for example. Supermassive black holes are thought to eject materials in the form of high speed jets and radio emissions.

Gravitational lensing was used to determine that a black hole had traveled between Earth and a star called MACHO-96-BL5.

These images show the brightening of MACHO-96-BL5 from ground-based telescopes (left) and the Hubble Space Telescope (right). — Gravitational lensing as a black hole traveled between Earth and star MACHO-96-BLG5. Courtesy of spacetelescope.org

According to spacetelescope.org the image above “Hubble Space Telescope looked at the object, it saw two images of the object close together, which indicated a gravitational lens effect. The intervening object was unseen. Therefore, it was concluded that a black hole had passed between Earth and the object.”

So Why Do We Study Black Holes?

Scientist study black holes to learn more about the early universe as some of these black holes were likely created very soon after the big bang. The idea that the laws of physics breakdown inside a black hole pose a challenge for scientists. If our known laws of physics don’t operate at the singularity then what is really going on inside a black hole? New theories such as loop quantum gravity are being investigated to see if they apply at singularities.

Shep Doeleman, senior research fellow at Harvard University and director of Event Horizon Telescope (EHT) stated in the Out There issue of Popular Science: “There’s no environment in the universe like a black hole. Being able to see such an object gives us a ‘natural laboratory.’ We can test long-standing theories about how objects move through space—like Einstein’s general relativity—by watching gravity-driven warps in spacetime impact how light travels. We can also study how black holes help shape the universe by sucking up matter.” And that’s why we study black holes!

Tags Black holes, escape velocity, event horizon, Event Horizon Telescope, law of universal gravitation, Schwarzschild radius

astronomy Betelgeuse cepheid variable physics solar system supernova

Supernovas and the Death of Red Supergiant Stars

Post author By Ngtriplett
Post date March 2, 2020
3 Comments on Supernovas and the Death of Red Supergiant Stars

The red supergiant, Betelgeuse has been in the news recently because of its unexpected behavior. Astronomers have noted that since October 2019 the red supergiant had been dimming and the star is now less than 40% of its normal brightness. Before we discuss what this means for the star let me introduce you to Betelgeuse.

Betelgeuse is between 640-724 light years from Earth and is the alpha or brightest star in the constellation Orion. Betelgeuse and Bellatrix make up the shoulders of Orion, the Hunter. Betelgeuse used to hold the distinction of being one of the top 10 brightest stars in the sky but has fallen to below 20th since it began dimming in October 2019. Betelgeuse is believed to be between 9 and 10 billion years old with a solar mass that is 12 times more massive than our sun.

find betelgeuse,betelgeuse in orion constellation — Image: Akira Fujii

Red Supergiants

So what is a red supergiant star? Red supergiants are stars of a specific size that are nearing the end of their lives. These stars spend only about 10% of their lives as red supergiants while the prior 90% is spent as a massive main sequence star. These stars have a mass greater than 10 solar masses meaning these stars have more than ten times the mass of our sun. In these stars most of the hydrogen fuel has been exhausted and the core stops producing energy and gravity causes the core to contract. The layer of the star surrounding the core contracts and heats up to a high enough temperature to start fusing hydrogen to helium. The outer parts of the star expand as a result of the star burning hydrogen. The star is producing more energy than necessary to offset the collapse due to gravity. The outer layer expands to several hundred solar radii and the surface temperature cools as a result of the increased surface area. This temperature decrease gives the star its reddish color.

The Dimming of Betelgeuse

Betelgeuse belongs to a class of stars called Cepheid variables or variables. These types of stars, according to universetoday.com “are essentially stars that experience fluctuations in their brightness (aka. absolute luminosity)”. So this means that some dimming from Betelgeuse is to be expected. Betelgeuse is considered to be a semi-regular variable star or slow irregular variable star which means its brightness or luminosity fluctuates in fairly predictable cycles. One cycle lasts for approximately 420 days, a second longer cycle lasts for close to six years, and a third cycle lasts somewhere between 100 and 180 days. The current reduction in brightness is larger than expected which has led to questions about what this means for the red supergiant. Some scientists think that this dimming is simply an extended dimming period lasting longer than the 420 day cycle while others speculate that Betelgeuse may be heading towards its ultimate demise, a supernova explosion. The European Southern Observatory (ESO) posted a video comparing the luminosity of Betelegeuse from December 2019 to January 2020, you can view the video here: https://www.youtube.com/watch?reload=9&v=o1ls7Gr9LTE According to ESO “this video shows the star Betelgeuse before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed.”

What is a Supernova and When Might it Occur for Betelgeuse?

According to nasa.gov, a supernova is “the explosion of a star. It is the largest explosion that takes place in space.” There is some speculation that Betelgeuse is nearing the end of its life and may go supernova in the near future. Let’s be clear about the meaning of “near future”. In our everyday life the “near future” may be a few days, a few weeks, maybe even a few months. Is astronomical terms, the “near future” may mean anywhere from a few thousand years to over a hundred thousand years. It is all based on your reference. Astronomers are comparing a few thousand years to the age of the universe which is estimated to be nearly 14 billion years old.

When the iron core reaches its Chandrasekhar mass which is about 1.4 times the mass of our sun or 1.4 Solar masses, the pressure of the core can no longer hold up against gravity and the iron core begins to collapse. During this collapse the electrons and iron nuclei get mashed together and electrons combine with protons in the nuclei to form more neutrons. This combing of electrons and protons results in a decrease in pressure which speeds up the collapse. The collapse of the core takes a mere few thousandths of a second.

As the core of the star reaches a size of around 31 miles the core contains a gas consisting of 70% neutrons and 30% protons with a temperature of 100,000,000,000 K. This object is now a proto-neutron star. The outer layers are continuing to fall into the core and a stream of neutrinos are flowing out of the proto-neutron star. This flow of neutrinos results in an enormous release of energy causing the outer layers of the star to be blown away. This flow of neutrinos is responsible for 99% or this enormous energy and light is the remaining 1% of the energy.

The brightness or luminosity of a supernova is 10 billion times greater than that of our sun. A supernova may outshine its own galaxy for several days. So why aren’t we seeing supernovas all the time? Shouldn’t we be able to detect them by the enormous luminosity? Scientists have found that core collapse supernova only occur at a rate of 1 supernova per century per galaxy.

Types of Supernova

There are several different types of supernova. They are classified by the type of light that is emitted by star. This can be thought of as the chemical signature of the star. These signatures help astronomers determine what elements are present in or created by the star.

The first is called type 1a supernova. Scientists believe that this type of supernova occurs when white dwarf stars, those stars who had masses less than 1.5 times the mass of our sun, acquire more mass than its internal pressure can withstand so it heats up and goes supernova. A star that became a white dwarf would not normally be massive enough to go supernova. The white dwarf is thought to have gained the mass from colliding with another white dwarf of from a companion red giant.

The next type of supernova is the type 1b supernova. In this case the star had a mass at least 25 times the mass of our own sun. It certainly is massive enough to go supernova. This type of star is thought to have shed material from its outer envelope later in life which is why there is little hydrogen in its spectrum. This type of supernova does show helium in its spectrum.

Type 1c supernova contain very little hydrogen and helium and are formed the same way the type 1b supernova are formed. The difference between type 1b and 1c is the lack of helium found in the 1c supernova.

Type II supernova or core collapse supernova contain large amounts of hydrogen and helium in its spectrum. It is believed that large stars with masses are larger than 8 times the mass of our sun undergo this type of supernova. The massive explosion results in the creation of a blackhole or a neutron star. This is the type of supernova a red supergiant, such as Betelgeuse, will undergo at the end of its life.

Types of supernovas. Courtesy of astronomy.swin.edu.au

Why Do We Care about Supernovas?

It turns out stars only produce elements up to iron through the fusion reactions in their core. So that begs the question, where do elements heavier than iron come from? Well, the extremely large amounts of energy and the enormous temperatures associated with supernova explosions can cause fusion of the heavier elements. These heavier elements are shot out through the universe during the supernova explosion. Many of the elements we have found here on earth were created in the core of a star during a supernova. The elements that travel through the universe eventually are used to create planets, new stars and anything and everything else in the universe. According to nasa.gov/audience/forstudents “one kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate.” So that’s why you should care about supernovas!

Elements heavier than iron that are formed in a supernova explosion.
Courtesy of herschel.jpl.nasa.gov

Tags astronomy, Betlegeuse, cepheid variable, physics, redsupergiant, supernova

astronomy physics solar system

The Mysterious Moon of Saturn: Enceladus

Post author By Ngtriplett
Post date February 24, 2020
9 Comments on The Mysterious Moon of Saturn: Enceladus

Image result for Enceladus — Saturn’s sixth largest moon Enceldaus. Courtesy of solarsystem.nasa.gov

The Cassini Mission

The Cassini orbiter was launched from Cape Canaveral, Florida on October 15th,1997. This was a joint effort between NASA, ESA (European Space Agency), and ASI (The Italian Space Agency). The mission was to study and explore Saturn and its system of rings and moons. The trip to Saturn would take seven years and the totality of the mission from the 1997 launch to 2017 plunge into Saturn would be 21 years. A majority, 13 years, of those years being spent studying the ringed planet and its moons. Saturn is approximately 75,000 miles in diameter which is nearly 10 times as large as Earth meaning 750 Earth sized planets could fit into Saturn. Saturn has a mass that is 95 times more massive than Earth. Saturn is roughly 1 billion miles from Earth, when the two are at their closest with respect to each other, which is 7 astronomical units (AU). One AU is the equal to the distance from the center of the Earth to the center of the Sun.

Discoveries of the Mission

The data, images, and samples collected by Cassini provided scientists with a wealth of information and knowledge about the ringed planet, the ring systems and its many moons. The mission revealed a large hexagon shaped jet stream at one pole of the ringed planet. “The hexagon is just a current of air, and weather features out there that share similarities to this are notoriously turbulent and unstable. A hurricane on Earth typically lasts a week, but this has been here for decades — and who knows — maybe centuries.” said Andrew Ingersoll of the Cassini Imaging Team. The Huygens probe made the first ever landing onto a moon in the outer solar system when it landed on Titan On January 14th, 2005. Titan was found to have rivers, lakes, and oceans. Titan has a thick nitrogenous atmosphere that may be similar to early Earth’s atmosphere. Six new moons were discovered to be orbiting Saturn which brings the total number of known moons orbiting Saturn to 82. With the discovery of these new moons Saturn now boast of having the most moons of any planet in our solar system.

Saturn’s Ring System

Image result for saturn ring structure — Courtesy of explanet.info

The breathtaking rings of Saturn were found to be made up of mostly frozen water mixed with space dust as well as rocky meteoroids. The particles making up the rings vary from the size of a grain of sand to the size of mountains. Vertical structures, which rise as high as 2 miles were seen on the B-ring of Saturn ring system. The rings reach an impressive 175,000 miles from the planet yet the average vertical height is only 30 feet in the main rings.

Image result for saturn's rings vertical structures — Image depicting the vertical structures of Saturn’s rings. Courtesy of universetoday.com

It turns out that the ringed planet did not always have these beautiful rings. This means the rings are younger than the planet itself. The rings are thought to have been created when asteroids, comets, or other bodies smashed into the planet sometime in the last 100 million years making the rings much younger than Saturn itself. The host planet for these rings was formed approximately 4.5 billion years ago. The Cassini mission also provided evidence that Saturn is losing its rings as the particles making up the rings are being pulled into the planet under the influence of Saturn’s strong magnetic field. Scientists predict that the ring structure will be gone in 100 million to 300 million years.

A Little about Enceladus

Saturn’s moon Enceladus, the sixth largest moon of Saturn, was discovered in 1789 by astronomer William Herschel. Enceladus is roughly 790 million miles from Earth and orbits its host planet, Saturn at an average distance of 148,000 miles. By way of comparison our moon orbits the Earth at an average distance of 238,000 miles. Enceladus is one-seventh the diameter of our own moon and much less massive than our moon. Here is a link comparing Enceladus to our own moon: https://solarsystem.nasa.gov/moons/saturn-moons/enceladus/by-the-numbers/

Enceladus boasts five distinct physical features including craters, cracks in the surface, ridges and fissures. Scientists speculate that the presence of cracks and ridges is evidence that the core of Enceladus is still a liquid composition. The smooth plains on the surface of the moon seem to indicate that there was water flowing from deep in the core to the surface. So how does the water make it from deep inside the moon to the surface? As it turns out there are a system of hydrothermal vents that connect to an ocean of saltwater below the surface. Scientists settled on this idea based on the presence of silica nanograins found in the E-rings. These nanograins can only be generated at temperatures above 90 degrees Celsius in areas where liquid water and rock interact. In 2005 scientists discovered that there were plumes of water shooting out from its south pole. These plumes are ejecting water which is continuously shot out into space at 800 miles per hour traveling hundreds of miles to the planets E-rings.

What’s so Special about Enceladus?

So what makes this moon special as compared to the other 81 moons orbiting Saturn? The major finding of Enceladus is not simply that it has an abundance of water, the moon Titan is believed to also have water, but it is what is contained in the plume that was surprising. Cassini was able to directly take samples of the plume from Enceladus as well as from the E-ring of Saturn. The results were stunning. The water contained a mixture of volatile gasses, water vapor, carbon dioxide, carbon monoxide, and organic materials. Similar results were obtained from both the samples taken from the E-ring of Saturn and the icy moon Enceladus.

“This is the first-ever detection of complex organics coming from an extraterrestrial water world,” said planetary scientist Frank Postberg from the University of Heidelberg in Germany. While no life forms have been detected yet scientists have not ruled out the possibility of finding life deep with in the moon or in the rings of Saturn. Scientists noted that most of the larger organic molecules were found in the E-ring. Speculation is that sunlight may have triggered chemical reactions in space which resulted in the complex organic molecules.

So What’s Next?

According to Potsberg the next logical step would be to go back to Enceladus “and see if there is extraterrestrial life.” Plans are already underway for a 2022 mission to the moons of Jupiter to search for habitable conditions on the icy moons containing subsurface oceans. With respect to returning to Saturn’s moon to search for life Potsberg notes “Nowhere else can a potentially habitable extraterrestrial ocean habitat be so easily probed by a space mission as in the case of Enceladus.” When the Cassini mission was close to termination a deliberate decision was made to send Cassini spiraling into the planet to avoid any accidental cross contamination with Enceladus. This suggests that a return trip to the moon was already being discussed and that the researchers wanted to leave no doubt that, if in fact, life was found on Enceladus it was not delivered by Cassini. Will the first extraterrestrial life be found on Enceladus? No one knows for sure but it is an extremely exciting possibility.

Image result for cassini final orbit — Courtesy of cosmosmagazine.com

Tags Cassini, Enceladus, Saturn, Voyager

astronomy Dark Matter physics

What is Dark Matter all About?

Post author By Ngtriplett
Post date February 17, 2020
2 Comments on What is Dark Matter all About?

You may have heard of the terms dark matter and if you read my post from last week then I know you have heard of dark energy. Let’s talk about dark matter, what it is and why it is called “dark”. We know a black hole is called black because it is not visible to us and even light can not escape its gravitational pull. So is dark matter called dark because we can’t physically see it? Well, yes actually. We are unable to detect it directly and are only able to infer its existence by the effects it has on other objects. So let’s take a closer look at what dark matter is and what it does.

What is Dark Matter?

Dark matter comprises about 27% of the universe and dark energy makes up about 68% of the universe which means that only 5% of the universe is comprised of matter as we know it. This “normal” matter is called baryonic matter and is the matter we traditionally think of. Protons, electrons, atoms, anything that makes up everything from people to stars are made from baryonic matter. Dark matter, like dark energy is called dark because we are unable to detect it directly. Dark matter does not interact at all with the electromagnetic spectrum which means it does not reflect light, absorb light, or emit anything we can detect. As of yet no one has been able to directly observe dark matter. Dark matter is thought to be a previously undiscovered subatomic particle that does not respond to the strong or weak nuclear forces.

The key point here is that when scientists account for all the visible or detectable mass in the universe it doesn’t add up to be enough to account for the gravitational effects we observe. As stated above only about 5 % of the universe is normal or baryonic matter so that means the most of the gravitational interactions in the universe are a result not of baryonic matter but of something else. We call this something else dark matter.

Image result for dark matter dark energy pie chart — Courtesy of mcdonaldobservatory.org

There are several ideas as to what actually comprises dark matter. Speculation includes dim brown dwarf stars, white dwarfs, neutron stars and even black holes. Some scientists have dismissed theses objects as dark matter candidates because the gravitational effect needed to make up the “missing mass” doesn’t match the gravitational effect observed by these objects. Others have stated that the “missing mass” may simply be normal baryonic matter that is simply more difficult to detect.

WIMPs and MACHOs

Weakly interacting massive particles or WIMPS are theoretical particles of non baryonic matter which have somewhere between 10 and 100 times the mass of a proton yet interact very weakly with normal or bayronic matter so they are difficult to observe. If WIMPS are what make up dark matter then there should be 5 times as many WIMPS as normal matter. We should be able to detect them as they do interact with normal matter and the sheer abundance of them should allow us to detect them through collisions with each other. So far no WIMPS have been discovered.

Massive astrophysical compact halo object or MACHO is another candidate for the composition of dark matter. These are objects composed of bayonic matter but are difficult to detect because they emit very little to no light. These include the neutron stars, supermassive black holes, and brown and white dwarfs as mentioned earlier. Because they emit so little light one way to detect them is through gravitational lensing which we will discuss a bit later. It appears that there are not enough of these objects throughout the universe to make up the “missing mass”.

Image result for WIMPS and MACHOS dark matter — Courtsey of slideplayer.com

Other Dark Matter Candidates

Neutrinos are particles that aren’t associated with and don’t interact with baryonic matter. Neutrinos stream from the sun and pass through all regular matter, including us all the time. They are difficult to detect as they do not interact with matter. A new type of neutrino is thought to make up dark matter by some in the scientific community. Sterile neutrinos are a theoretical type of neutrino that have been proposed, they only interact with baryonic matter via gravity.

The Kaluza-Klein particle is a theoretical particle that would interact with the electromagnetic spectrum as well as gravity which should make them easy to detect. These particles are thought to exist in the fifth dimension making them difficult but not impossible to detect. The Kaluza-Klein particle is predicted to decay into particles we can readily observe, such as neutrinos and photons. As of yet though, none of these exotic particles have been observed.

How Do We Know Dark Matter Exists if We Can’t See It?

There are three pieces of evidence used to prove the existence of dark matter even though we can not detect it directly. The first is the speed of stars rotating on the outside edge of spinning galaxies. The stars on the outer edge should move at a much slower rate than those close to the center where most of the baryonic mass of the galaxy is contained. Direct observation has shown that these stars, at the outer edge of the galaxy, are moving at a rate very close to the rate of stars closer to the center. This has led scientists to the conclusion that there must be some mass distributed throughout the galaxy exerting a gravitational effect on the stars farther out from the center.

Stars, as it turns out, orbit their parent galaxy. By using Newton’s equations of force and the Universal Law of Gravitation we know that the force that causes the star to orbit in a circular orbit are equal to the force due to gravity on the star. If these forces were not equal then the star would careen into the center of the galaxy or fly off into space. Close to the center of a galaxy these forces are approximately equal as expected. Stars farther out from the center don’t appear to have these forces equal. So there has to be something going on to keep these stars in orbit. Dark matter is one such explanation.

Image result for dark matter graph — The graph depicts the discrepancy between the expected velocity (A) and actual velocity (B) of a star as the distance from the center of the galaxy, where most of the baryonic matter is located, increases. Courtesy of popscicoll.org

Gravitational lensing is a well documented phenomena in which massive objects distort the fabric of space time. Light must travel along this fabric and if there are massive objects distorting this fabric then the the source of light may appear shifted from the actual position as a result of distortions in space.

Image result for gravitational lensing — A visualization of gravitational lensing. Courtesy of agitatorgallery.com

Gravitational lensing as a result of dark matter. Courtesy of nasa.gov

The third piece of evidence supporting dark matter is the the Bullet Cluster Galaxy merger. Two galaxies collided and due to only about 2% of a galaxy being made up of stars and roughly 5-15% being made up of gas and plasma there is a low probability of any baryonic matter colliding with one another. After the merger of the two galaxies gravitational lensing of background objects allowed scientists to determine where the accumulations of mass were located. It turns out that the dark matter was separated from baryonic matter in large enough quantities to cause gravitational lensing.

Image result for bullet cluster gravitational lensing — Courtesy of nanoqed.org

What if the Missing Mass is the Result of Something Else?

Not everyone is ready to accept that dark matter is responsible for the “missing mass” problem in the universe. Some scientists believe the problem isn’t that mass is missing but rather our theories and equations are incorrect. One particular idea is that Newton’s laws require a modification so that they match the observed behavior of galaxies. Those that support this idea have developed MOND or Modified Newtonian Dynamics as an alternative to dark matter. This theory suggest in situations when the acceleration rates are low Newton’s laws do not accurately describe the motion of galaxies. This alternative to dark matter is as still being developed and refined to better match observation.

Image result for dark matter theory — Courtesy of quantamagazine.org

Scientists know that the amount of baryonic matter which they are able to detect is not enough to hold galaxies together or keep stars from flying off into space rather than orbiting their parent galaxy. What is unknown is the precise mechanism causing the behavior observed by galaxies and stars. Dark matter is certainly the most widely accepted theory about our “missing mass” problem but it is by no means the only theory.

Tags astronomy, cosmology, Dark Energy, Dark Matter, galaxies, gravitational lensing, MACHOs, Modified Newtonian Dynamics, physics, WIMPs